This invention relates to preparation of cyclohexanone by hydrogenation of phenol; more particularly, it relates to a computer controlled process for preparation of cyclohexanone by liquid phase, catalytic hydrogenation of phenol in three or more hydrogenation stages.
In the hydrogenation of phenol employing a palladium catalyst, the activity of the catalyst, and hence the rate of hydrogenation, decreases with continued use of the catalyst due to impurities present in the hydrogenation reaction mixture which poison the catalyst. While processes, such as those disclosed in U.S. Pat. Nos. 3,692,845 and 3,187,050, have been developed to purify organic compounds such as phenol to be hydrogenated, the poisoning of metallic catalysts has not been entirely eliminated in large scale commercial processes due to long term accumulation of impurities, particularly impurities which are produced during the processing.
To avoid the economically prohibitive alternatives of discarding poisoned catalyst or continuing to use the poisoned catalyst at a reduced rate of hydrogenation, it is known to promote the rate of hydrogenation, thereby at least partially overcoming the disadvantages of continued use of such poisoned palladium catalysts. The hydrogenation of phenol to cyclohexanone is normally promoted by the use of "promoted palladium-on-carbon catalysts", i.e., catalysts which have been treated prior to their addition to the hydrogenation reaction mixture, to incorporate on the catalysts a material which enhances the activity of the catalysts. Thus, in U.S. Pat. No. 3,076,810, cyclohexanone is produced by hydrogenating phenol using a sodium-promoted palladium catalyst which has been modified prior to its introduction to the reaction mixture to incorporate sodium thereon. Alkaline reacting agents in limited amounts are also disclosed as being added to assist in promotion when the sodium-promoted catalyst is employed.
U.S. application Ser. No. 815,397, filed July 13, 1977, discloses a highly active catalyst for selective hydrogenation of phenol to cyclohexanone which comprises 0.2 to 10 weight percent of palladium, based on the total weight of the catalyst, supported on carbon particles having diameters of 3 to 300 microns and a surface area of 100 to 2000 m.sup.2 /gram, said catalyst being promoted by sodium in an amount of at least 1000 ppm. Preferably, said sodium-promoted palladium catalyst is additionally promoted during said hydrogenation by contacting the catalyst with phenol containing a small amount of an in situ promoter selected from the group consisting of sodium hydroxide, sodium carbonate, and sodium phenate, said amount being 10 to 300 ppm in terms of sodium of said in situ promoter.
The highly active catalyst of U.S. application Ser. No. 815,397 is an important contribution to this art because it permits hydrogenation of phenol with reduced amounts of catalyst and with intrinsic safety by operating at temperatures at or below the atmospheric boiling point of the reaction mass. However, we have found that control of the hydrogenation reaction is difficult with use of the highly active catalyst, and research has been continued to develop a computer controlled process.